A superabsorbent polymer (SAP) is a type of synthetic polymeric materials capable of absorbing moisture from about 500 to 1000 times its own weight, and also called SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields or the like.
As a preparation process for such superabsorbent polymers, a process by a reverse phase suspension polymerization and a process by a solution polymerization have been known. Of them, preparation of the superabsorbent polymer by reverse phase suspension polymerization is disclosed in, for example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, S57-198714, etc. Further, preparation of the superabsorbent polymer by the solution polymerization further includes a thermal polymerization method in which a water-containing gel polymer is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays onto a belt to be polymerized and dried at the same time.
Meanwhile, absorption rate, one of important physical properties of the superabsorbent polymer, is associated with surface dryness of products in contact with the skin, such as diapers. Generally, absorption rate may be improved by increasing surface area of the superabsorbent polymer.
For example, a method of forming a porous structure on the particle surface of the superabsorbent polymer by using a foaming agent is applied. However, since it is difficult to form a sufficient amount of the porous structure by a general foaming agent, there is a drawback that the absorption rate is not greatly increased.
The method of forming the porous structure on the particle surface by using a foaming agent is as described in a literature (Kabiri, K., Omidian, H. and Zohuriaan-Mehr, M. (2003), Novel approach to highly porous superabsorbent hydrogels: synergistic effect of porogens on porosity and swelling rate. Polym. Int., 52: 1158-1164). As various factors influencing pore formation, the literature suggests gelation time in polymerization as well as selection of a proper porogen, and porogen injection time and method. Carbonate-based materials are suggested as foaming agents generally known, and a time point of injection of these foaming agents is suggested as an important factor of forming the pore structure.
Another example is a method of increasing surface area by re-granulating powder obtained in the preparation process of the superabsorbent polymer to form non-uniform porous particles. This method may be used to improve absorption rate of the superabsorbent polymer, but there is a limitation in that centrifuge retention capacity (CRC) and absorbency under pressure (AUP) of the polymer become relatively low. Like this, there is a trade-off between physical properties of the superabsorbent polymer such as absorption rate, centrifuge retention capacity, absorbency under load, etc. Accordingly, there is an urgent demand for a preparation method capable of improving these physical properties at the same time.